Life Cycle Analysis (LCA) is a technique used to measure and compile all factors relating to the production, usage, and disposal of a fuel or product. But, says Smith and his team, LCAs are overestimating the positive aspects of biofuel use ignoring the emission of CO2 by vehicles that use ethanol and biodiesel, even when there's no good reason for this.

The justification for ignoring this CO2 is that plants grown for biofuel absorb and therefore offset the same amount of carbon as is emitted by refining and combusting the fuel.

But this isn't right, says Smith, as it double-counts the carbon absorbed by plants when bioenergy crops are grown on land already used for crop production or growing other plants.

"The best opportunity to make beneficial biofuels is to use waste material or to focus on relatively wet but highly degraded land," he says.

And the effect of this miscalculation is magnified, says Smith, when it's combined with the underestimation of nitrogen emissions from fertilizer application.

"Emissions of N2O from the soil make a large contribution to the global warming associated with crop production because each kilogram of N2O emitted to the atmosphere has about the same effect as 300kg of CO2," he says.

He believes that several current LCAs underestimate the percentage of nitrogen fertilizer application that is actually emitted to the atmosphere as a greenhouse gas. Indeed, he says, real-world observations show that the true value is nearly double that used in the models, making them highly unreliable.